Fuelling our future
We are currently heavily dependent on crude oil derived fuel sources such as petrol and diesel. There are three serious problems with this. Firstly, these reserves are made by squashing natural materials like plants under pressure over millions of years, so once they run out, we can’t quickly make more. Secondly, these fuels contain lots of carbon and when this is burnt with oxygen in an engine, it releases carbon dioxide, a greenhouse gas threatening our planet with rising global temperatures. Finally, obtaining the oil source naturally means that it also contains some other elements which release harmful pollutants when they are burnt, such as oxides of nitrogen and sulphur. Both of these gases lead to acid rain, which has devastating effects on the environment.
It is clear that the age of fossil fuel dependency is over and we need to find new ways to power our energetically demanding lives.
The clean energy challenge
It takes time to develop new technologies and build the supporting infrastructure. Renewable energy from wind and solar power, for example, present exciting and sustainable opportunities; tapping into free, abundant resources without releasing harmful gases. Government grants, such as the 2010 UK feed-in-tariff, helped to accelerate individual investment in solar panels. However, change in government policy and high technology cost coupled with the low efficiency and large space requirement limits immediate widespread use. As such, these technologies represent a long-term future solution.
But, we need cleaner energy solutions now and, according to Professor Robert Tooze, Director of Drochaid research services, “we have to recognise that we’re in a transition period for energy.” He adds, “we need to manage the transition to some future beyond hydrocarbon fuels, and what we can do in the meantime is to make the cleanest fuels we can”.
Synthetic liquid fuels
Artificially producing fuel from sustainable carbon sources, in so-called synthetic liquid fuels, is a potential solution to our energy crisis.
Synthetic liquid fuels are made by sticking together carbon atoms, like coal or natural gas, until the chain is long enough to match petrol or diesel. The synthetically produced fuel can be used directly in our vehicles so there is no time delay for developing new infrastructure or engines. Plus, these artificial fuels release less harmful pollutants like sulphur and alleviate political crude oil dependence.
History of synthetic liquid fuels
“If you look back in history to synthetic liquid fuels it’s more about necessity being the mother of invention”, Tooze said. A century ago, Germany, who does not have their own crude oil reserves, identified the weakness of fossil fuel dependency in a war scenario. So in the 1920s, two German chemists, Franz Fischer and Hans Tropsch, set to work to convert their coal reserves into a usable, man-made liquid fuel, in a process coined Fischer-Tropsch. Tooze added, “the beauty of a synthetic fuel process is it doesn’t really matter the original form of carbon that you feed to this process.” By 1936, the process had been commercialised, by Braunkohlen Benzin AG, and was used by the Germans in the Second World War to make alternative transportation fuels.
Since then, other coal-rich countries have adopted this relatively simple technology, but the cost of building the huge industrial plants represents a significant barrier to entry.
A company called Sasol in coal-rich South Africa have been at the forefront of developing commercial synthetic liquid fuels and opened their first commercial plant in the 50s. One of their joint ventures in Qatar, Oryx GTL produces 34,000 barrels of fuel per day. And the well known fuel company Shell has been investing in synthetic liquid fuels since the 1970s, with their first Bintulu plant in Malaysia producing 12,000 barrels per day. Another plant, Pearl GTL, jointly owned by Shell and Qatar Petroleum, is the largest plant of its kind and converts natural gas or methane into liquid fuels, with an output of 140,000 barrels per day. That’s enough fuel from one day at those three plants to power a medium-sized car for roughly 52 million miles (assuming 280 driving miles from one barrel and 17 miles per gallon fuel efficiency).
How to make synthetic liquid fuels: The Fischer-Tropsch process
There are three main stages required to convert a carbon source into a liquid fuel:
1. A carbon source such as coal or natural gas is heated to high temperatures to produce carbon monoxide and combined with hydrogen gas to make a mixture of gases called synthesis gas.
2. The gas mixture is passed over a compound called a catalyst to speed up the reaction of sticking the carbon atoms together, producing the long carbon chain, which is the fuel. The reaction takes place at roughly 200 °C under high pressure.
3. Some processing steps are carried out to obtain the final version of the fuel, which is fed into your car.
A huge amount of research and development has been invested into improving this process in order to make it commercially viable and as efficient as possible. It is now considered a mature technology but there is nevertheless, room for improvement to increase efficiency and ultimately reduce operating costs. An area of continued research interest is the catalyst.
The catalyst used in the second stage speeds up the chemical reactions between the carbon monoxide and hydrogen gas molecules, which would not take place fast enough without the catalyst. It effectively acts as a specialised pocket for the gas molecules to react within.
The clever catalyst molecules are so important in industry because, crucially, time is money. Why wait around for your product to form slowly if you could use a catalyst to make the product more quickly and therefore make money faster? Due to the significant cost and efficiency benefits of using a catalyst, this is an area for continual optimisation and as such attracts a significant amount of research interest, both in industry and academia.
Cobalt and iron metals are used as the catalyst to make synthetic liquid fuels. The choice of metal depends on several factors such as the nature of the initial carbon source with cobalt used for natural gas and iron used for coal. In addition, different metals favour the formation of different carbon chain lengths so the catalyst selection helps to tailor the fuel product obtained. But with the cost of cobalt around 300 times higher than iron, the use of this metal has to be justified in terms of an overall economical benefit. Typically, this is achieved because cobalt catalysts are more selective to make the desired fuel product rather than unwanted, polluting side products like carbon dioxide.
Professor Emiel Hensen at Eindhoven University of Technology, works on synthetic liquid fuels and specifically focuses on designing better catalysts. Recently an international team lead by Hensen published exciting results about an active catalyst made of iron mixed with carbon that was previously thought to be unstable and ineffective at speeding up the chemical reaction. Usually the catalyst encourages the formation of a lot of undesired carbon dioxide but the special new form they made avoids this problem. Due to the lower cost of iron catalysts, discovery of efficient active and selective iron catalysts is industrially relevant and of commercial importance.
However, new results in the laboratory take a while to translate to a catalyst that can be used in an industrial process. Not least because the process to make the catalyst needs to be scaled up, which is not always straightforward. Furthermore, an industrial catalyst also contains various other metal components to give it even more properties. Catalysts used on an industrial scale need a long lifetime so it does not need to be regularly replaced, justifying the cost.
In addition to improving the efficiency of the catalyst used to speed up the process of making synthetic liquid fuels, other challenges to the widespread implementation of synthetic liquid fuels exist.
One of the most significant challenges to producing synthetic liquid fuels is the price competitiveness compared to traditional crude oil-derived fuels. If the price of oil is low, synthetic liquid fuels appear too expensive but the process is economically viable after the oil price is above a certain price per barrel. “The typical price for coal and gas to liquid [fuel] is around $70 to $100 per barrel, so the price of oil should be higher than that” said Hensen. Fluctuating oil prices, therefore, makes the significant initial capital investment of a synthetic liquid fuels plant fairly risky. However, it can also be incredibly lucrative. For example, the Oryx GTL plant was making $500 million profit after 3 years of operation.
Despite numerous examples of huge industrial plants converting carbon sources into liquid fuels, Hensen said that for synthetic liquid fuels “if you add all the capacity nowadays of gas to liquid and coal to liquid, it’s half a percent of the total crude oil used.” This is understandable when you consider that the Pearl GTL plant cost $18 billion to build and highlights just how much petrol and diesel we use.
The field of synthetic liquid fuels is broadly referred to as C1 chemistry because it takes molecules with one carbon atom i.e. C1 and converts them into carbon-based products with more than one carbon atoms. This type of chemical transformation can be exploited to make the less polluting fuels discussed so far. In fact, some of the synthetic liquid fuels produced nowadays are blended with oil-derived diesel to reduce the harmful emissions and improve the fuel properties.
But applications of this carbon-linking chemistry are not limited to the energy sector. China, for example, has been constructing large industrial plants to use this process to convert their abundant coal reserves into the building blocks needed to make plastics.
The process of joining carbon sources containing individual atoms into longer carbon chains could be considered a renewable process if the carbon source is carefully selected. For example household food waste can be processed to make the carbon monoxide needed for synthesis gas. If this waste is the gas source then the fuel obtained is renewable in terms of its acquisition, however there is still the problem of the carbon dioxide released when the renewable, synthetic liquid fuel is burnt in an engine. Imagine if we could use this released carbon dioxide and feed it back into the synthetic liquid fuel process to remake our fuel. In terms of the uptake of synthetic liquid fuel technologies Hensen said “whether it will increase is uncertain,” but he added, “we are dreaming of a circular economy”. Hopefully enough innovative research will make this dream a reality. Certainly in the meantime, until we can successfully store energy from renewables like wind and solar, gas or coal to liquid fuel technology is a viable option as an intermediate fuel. This class of cleaner fuels are seen as particularly useful to reduce pollution from industries that are intrinsically hard to electrify such as air travel.